The present invention relates to variable pressure fuel systems for a carbureted internal combustion engine.
The fuel system of a carbureted internal combustion engine includes a fuel pump and a carburetor. The fuel pump boosts the pressure of the fuel and supplies the pressurized fuel to the carburetor. The carburetor has a needle valve controlled by a float in a float bowl. When the level of fuel rises in the float bowl, the float buoyed by the fuel acts on the needle valve and tends to close it. Fuel is drawn from the float bowl into a venturi of the carburetor by a pressure differential between the float bowl and the throat of the venturi. With increasing height of the fuel in the float bowl, the pressure differential tending to force a fuel into the carburetor venturi increases. Theoretically, when the fuel level in the float bowl reaches a predetermined height the float closes the needle valve.
The fuel pump delivery pressure tends to unseat the needle valve and admit fuel into the carburetor float bowl. When the pressure tending to force fuel into the venturi increases, the fuel level in the float bowl increases and an excessive amount of fuel can be delivered to the venturi. The engine will then run fuel-rich. Fuel economy suffers. The higher the fuel pressure, the greater the height of fuel in the float bowl necessary to increase the force of the float sufficiently to close the valve. Quite often the height of fuel in the float bowl at less than full load produces a richer fuel-to-air ratio than is required for satisfactory engine operation. Vibration and inertial effects on the needle valve can unseat the valve even when fuel height in the float bowl is at its theoretical correct value. An excess amount of fuel then enters the float bowl.
Fuel pump delivery pressure is designed for the maximum fuel consumption rate requirements of an engine. A typical pressure is 6 p.s.i.g. Obviously, when operating at less than maximum load the required rate of fuel delivery drops off accordingly.
Under full load, fuel will be delivered rapidly to the carburetor. When there is a sudden drop in load, the carburetor can "load up" because the inertia of the fuel in the delivery line adds to the fuel pressure to keep the needle valve open too long.
Excessive fuel pressure can also lead to percolation and hard start problems in hot weather. With excess fuel in the float bowl and a high vapor pressure, the fuel tends to percolate out of the bowl. The high vapor pressure adds to the excessive height of fuel in the float bowl to create an excessively fuel-rich condition that makes starting difficult.
The problem of excessive fuel pressure at a carburetor has been recognized in the art. U.S. Pat. No. 2,737,167 to Dickey proposes to solve the problem by a system that meters fuel proportionally to engine demand over the full range of demand possibilities. U.S. Pat. No. 2,905,455 to Eberhardt provides low fuel pressure at idle and high fuel pressure at all other speeds. U.S. Pat. No. 3,039,485 to Brohl, like Dickey, describes a proportioning system. With increasing load, fuel pressure increases. Load is sensed by manifold vacuum acting on a diaphragm.
Proportional systems can still supply too much fuel because of excessive delivery pressure to the carburetor, and, therefore, these systems result in improved but not optimized fuel economy.
Those systems employing a proportional increase in fuel pressure do not completely overcome the problem of the needle valve opening when it should not because fuel pressure acting on the needle valve tending to open it increases with load. In other words, if the fuel pump delivery pressure is maintained at a relatively low value over most of the operating range of an engine and stepped up only on occurrences of high demand, a larger margin exists over those systems that are proportional. In addition, those systems that employ a proportional system operated from engine manifold on vacuum risk the admission of fuel directly into the manifold upon failure of the diaphragm, and risk engine fire. Moreover, there are different fuel pump output pressures for different types of fuel pumps. For a device of universal application to apply with different fuel pumps and different output pressures, the device must be tolerant of these different output pressures.
It has also been a practice to reduce the pressure of the fuel to the needle valve to a single low value.
This has not been satisfactory in that the fuel at a single reduced pressure results in the fuel in the carburetor bowl being held at a single low level. When an engine is under a substantial load, being accelerated, or being started, it is desirable to have a relatively high fuel level in the bowl to provide a richer mixture.